Road safety barriers are designed to shield motorists from impacts with roadside hazards such as trees, utility poles, steep embankments, culverts and other vehicles. Road safety barriers are typically classified into three categories: rigid barriers; flexible barriers; and semi-rigid barriers.
Rigid barriers are generally employed where there is minimal margin for vehicles to move outside allocated traffic lanes. Such barriers may, for example, be formed of solid concrete. A disadvantage of concrete barriers is that vehicles that collide with the barriers could face severe impact damage.
Flexible barriers are employed where there is significant space beside the roadway to accommodate movement of a vehicle outside the traffic lanes. One such flexible barrier is formed of a series of tensioned cables.
Where there is lesser degree of available space adjacent a roadway, a semi-flexible barrier will generally be employed. One such barrier includes a series of upright posts along the roadside that support a guardrail. The guardrail serves as a containment ribbon under impact to absorb and re-direct a vehicle. During impact, the support posts may be knocked down and the guardrail needs to be maintained at a suitable height, to provide the required containment function, instead of being dragged down and under the vehicle as a result of continued attachment to the posts.
A number of mechanisms have been used in an attempt to maintain the height of the guardrail during impact. A first mechanism is to introduce a spacer block between the supporting posts and the guardrail. The spacer block rotates over the post as the post is knocked down to keep the guardrail elevated, until the guardrail physically disengages from the post. The spacer block minimized snagging of the vehicle on the post, due to the spacing between the guardrail and the posts, and reduces the potential for the vehicle to vault over the guardrail as the post height is maintained during the initial stages of impact. However, a problem with the spacer block arrangement is that the posts and spacer blocks are generally formed of open steel C-sections to minimize costs and both the posts and blocks tend to plastically crush and deform on impact instead of deflecting in order to separate from the guardrail. This ultimately has the effect of dragging the guardrail under the vehicle as a result of the vehicle fully impacting with the barrier.
Another mechanism to facilitate separation of the guardrail from the posts is to fix connecting collars to the guardrail. The collars are designed as sliding carriages that are arranged to slide over the posts and locate the guardrail at an appropriate height. When a vehicle impacts the barrier, the posts collapse and the collars freely slide off the ends of the posts. Once the collars are released, the guardrail becomes a substantially flexible ribbon that then demands a considerable distance be provided adjacent the roadway to arrest and redirect a vehicle.
Another mechanism is to attach the guardrail directly to the supporting posts by countersunk bolts that have a specific tapered head designed to pull through the guardrail under load. However, this mechanism has not been widely implemented since cheaper, non-tapered fasteners are readily available substitution may occur. If cheaper fasteners are installed, the guardrail will likely not function appropriately in the event of an impact. For safety reasons, this mechanism is not widely endorsed.
Any reference to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the date of this application.